1. Field of the Invention
The present invention relates to wireless communications. More specifically, the present invention provides a method that efficiently synchronizes a sub-network base station to a macrocell network over-the-air and minimizes interference.
2. Discussion of the Related Art
A femtocell is a new type of cell in cellular communication that is located within the premises of a mobile user. A femtocell offers numerous advantages both for the operator and the consumer, such as increased data rates, improved indoor coverage, and decreased backbone traffic for the operator. Advantages of femtocells are disclosed in the articles: (a) “Effects of user-deployed, co-channel femtocells on the call drop probability in a residential scenario,” by L. T. W. Ho and H. Claussen, published in Proc. IEEE Int. Symp. Personal, Indoor, Mobile Radio Commun. (PIMRC), Athens, Greece, September 2007, pp. 1-5; (b) “Performance of macro- and co-channel femtocells in a hierarchical cell structure,” by H. Claussen, published in Proc. IEEE Int. Symp. Personal, Indoor, Mobile Radio Commun. (PIMRC), Athens, Greece, September 2007, pp. 1-5; and (c) “3rd generation partnership project; technical specification group radio access networks; 3G Home NodeB study item technical report,” the 3GPP standard, Shangai, China, March 2008, 3GPP TR 25.820 V8.0.0 (2008-03). [available at http://www.3gpp.org/ftp/Specs/html-info/25820.htm. However, femtocells have a number of unique technical challenges, such as co-channel interference received from both the macrocell and other femtocells, hand-off and security issues.
In an orthogonal frequency division multiple access (OFDMA) based femtocell network, the femtocell may co-exist with the macrocell network by using the macrocell's unused subcarriers. In such a femtocell, as illustrated by FIG. 1, uplink (UL) signals from macrocell mobile stations (mMSs) may arrive at the femtocell base station (fBS) at different delays. As shown in FIG. 1, uplink interference from macrocell users (i.e., mMSs 101-104) are synchronized to macrocell base station (mBS) 110. However, the signals of mMSs 101-104 arrive at fBS 120 after different delays, resulting in inter-channel or inter-carrier interference (ICI).
As discussed in the article “Opportunity detection for OFDMA systems with timing misalignment” (“Sahin”), by M. E. Sahin, I. Guvenc, M. R. Jeong, and H. Arslan, presented in the IEEE Global Telecom. Conf. (GLOBECOM), New Orleans, La., November 2008, when the signal from an mMS arrives after the cyclic prefix of a femtocell user, the signal from the mMS causes considerable ICI. To prevent or reduce ICI from UL signals of a macrocell network, an fBS should preferably efficiently synchronize to the macrocell network. However, because the mMSs do not communicate directly with a femtocell, the mMSs do not transmit any pilot/training symbols to the femtocell for synchronization. Therefore, a femtocell has to “blindly” synchronize to the macrocell network through an UL received multiuser signal.
Blind time synchronization in OFDM systems are disclosed, for example, in the following articles: (a) “Blind symbol-timing and frequency-offset estimation in OFDM systems with real data symbols” (“Tanda”), by M. Tanda, published in IEEE Trans. Commun., vol. 52, no. 10, pp. 1609-1612, October 2004; (b) “Performance comparison of blind symbol timing estimation in cyclic prefixed OFDM systems” (“Guo”), by H. Guo, Q. Cheng, and R. Liyana-Pathirana, published in Proc. IEEE TENCON, Melbourne, Australia, November 2005, pp. 1-5; (c). “ML estimation of time and frequency offset in OFDM systems” (“Beek”), by V. Beek, M. Sandell, and P. Borjesson, published in IEEE Trans. Sig Processing, vol. 45, no. 7, pp. 1800-1805, July 1997; (d) “On the optimality of metrics for coarse frame synchronization in OFDM: A comparison” (“Muller”), by S. Muller-Weinfurtner, published in Proc. IEEE Int. Symp. Personal, Indoor, Mobile Radio Commun. (PIMRC), Boston, Mass., September 1998; (f) “Frame synchronization OFDM systems in frequency selective fading channels” (“Speth”), by M. M. Speth, F. Classen, and H. Meyr, published in Proc. IEEE Vehic. Technol. Conf. (VTC), Phoenix, Ariz., May. 19977, pp. 1807-1811; (g) “A novel blind carrier synchronization method for MIMO OFDM system,” by D. Wang, J. Wei, and X. Zhang, published in Proc. IEEE Military Commun. Conf. (MILCOM), Orlando, Fla., October 2007, pp. 1-4; (h) “A blind uplink OFDM synchronization algorithm based on cyclostationarity,” by M. Hua and J. Zhu, published in Proc. IEEE Vehic. Technol. Conf. (VTC), vol. 2, Stockholm, Sweden, June 2005, pp. 1002-1006; (i) “Blind time and frequency synchronization in OFDM based communication,” by H. W. Kim, S. min Lee, K. Kang, and D.-S. Ahn, published in Proc. IEEE Vehic. Technol. Conf. (VTC), Melbourne, Australia, September 2006, pp. 1-5; and (j) “Blind OFDM symbol synchronization in ISI channels,” by R. Negi and J. M. Cioffi, published in IEEE Trans. Commun., vol. 50, no. 9, pp. 1525-1534, September 2002. The techniques disclosed in these references perform synchronization to decode the received signal, but not for reducing or minimizing ICI between the macrocell and the femtocell.
However, blind synchronization for the multiuser case (i.e., synchronization with the UL multiuser signal) has not been described in the literature. While narrowband interference effects on blind synchronization are disclosed, for example, in “Analysis of the narrowband interference effect on OFDM timing synchronization,” by M. Marey and H. Steendam, published in IEEE Trans. Sig. Processing, vol. 55, no. 9, pp. 4558-4566, September 2007, the system considers a pilot-aided timing estimation approach, rather than a blind estimation approach. As narrowband interference is different from multiuser interference, applicable estimation techniques may be different.
In the article, “A time and frequency synchronization scheme for multiuser OFDM,” by J. V. Beek, P. O. Boijesson, M. L. Boucheret, D. Landstrom, J. M. Arenas, P. Odling, C. Ostberg, M. Wahlqvist, and S. K. Wilson, published in IEEE J. Select. Areas Commun. (JSAC), vol. 17, no. 11, pp. 1900-1914, November 1999, a time/frequency synchronization scheme was disclosed for a multiuser OFDM system. However, in that system, each user is specifically separated from other users (through separation filters in the frequency domain) for the purpose of estimating each user's time/frequency offsets individually.
The purpose of a blind synchronization in a femtocell is not for decoding the received signal. Rather, efficient synchronization with the macrocell UL multiuser signal allows the femtocell to reduce ICI. As mentioned above, each mMS is typically synchronized to its own mBS. See, e.g., the initial/periodic ranging mechanism in the IEEE 802.16e standard (i.e., the “WiMAX standard”), February 2006, available at http://www.ieee802.org/16/pubs/80216e.htm. Synchronization between the mMSs with the mBS impacts the statistics of signal arrival times from mMSs to the fBS.